1. Field of the Invention
This invention relates to molten carbonate fuel cell anodes incorporating a sulfur tolerant carbon monoxide to hydrogen water-gas-shift catalyst and a process for improved molten carbonate fuel cell operation using fuel gas mixtures of over 10 volume percent carbon monoxide. The anodes and process of this invention are particularly suitable when using synthetic gas as a fuel, which gas frequently contains sulfur containing chemicals such as hydrogen sulfide.
2. Description of the Prior Art
The use of molten carbonate fuel cells is well known for the conversion of chemical energy directly into electrical energy by a galvanic oxidation process. Molten carbonate fuel cells generally comprise two electrodes with their current collectors, a cathode and an anode, an electrolyte tile making contact with both of the electrodes and a cell housing to physically retain the cell components. Under fuel cell operating conditions, generally about 500.degree. to about 700.degree. C., the entire electrolyte tile, the carbonate and the inert support material, forms a paste and thus the electrolyte diaphragms of this type are known as "paste electrolytes". The electrolyte is in direct contact with the electrode where the three-phase, gas-electrolyte-electrode, reactions take place. At temperatures below about 650.degree. C. hydrogen is essentially the only electrochemically active material and is consumed in the anode zone according to the reaction: EQU CO.sub.3.sup.= +H.sub.2 .fwdarw.H.sub.2 O+CO.sub.2 +2e.sup.-
The electrons flow to the cathode through an external circuit producing the desired current flow and electrical balance. The carbonate ions are produced in the cathode zone by the reaction: EQU 2e.sup.- +1/2O.sub.2 +CO.sub.2 .fwdarw.CO .sub.3.sup.=
The carbonate ions are transferred through the electrolyte to the anode zone. At temperatures above about 750.degree. C. carbon monoxide becomes electrochemically active in the anode zone. Further details of construction and operation of high temperature carbonate fuel cells is set forth in U.S. Pat. Nos. 4,009,321 and 4,247,604 and the references referred to therein, all incorporated herein by reference.
One desirably used fuel for molten carbonate fuel cells is a mixture of gases comprising principally hydrogen, carbon dioxide, and carbon monoxide as obtained by gasification of naturally occurring carbonaceous material such as coal, shale or peat, as well known in the art. Gas mixtures obtained by these processes usually contain sulfur contaminants, such as hydrogen sulfide. Thus, when such gaseous fuel mixtures are supplied to the fuel zone of a molten carbonate fuel cell operated at below about 650.degree. C. and the fuel mixture comprises typically 10 to 35 volume percent carbon monoxide, representing up to about 50 percent of the fuel content of the fuel gas, low fuel utilization is obtained with only hydrogen being electrochemically active in the molten carbonate fuel cell under such conditions. The initial carbon monoxide to hydrogen water-gas-shift catalytic activity of a conventional nickel molten carbonate fuel cell anode for production of additional hydrogen is quickly poisoned by the presence of sulfur containing chemicals. Smith, S. W., Kunz, H. R, Vogel, W. M. and Szymanski, S. J., "Effects of Sulfur on Molten Carbonate Fuel Cells", (paper presented at Electrochemical Society Meeting, Montreal, Canada, May 1982), have reported complete failure of cell voltage in a molten carbonate fuel cell with a nickel anode when 2 ppm hydrogen sulfide, on a volume basis, was present in the fuel gas. The reduction in molten carbonate fuel cell performance to unsatisfactory levels with the presence of very small amounts of hydrogen sulfide in the fuel gas has been reported by several investigators: United Technologies Corporation, "Development of Molten Carbonate Fuel Cell Power Plant Technology", DOE/ET/15440-8 Quarterly Technical Progress Report No. 8, prepared for Department of Energy, under contract No. DE-AC01-79ET15440, February 1983; Vogel, W. M. and Smith, S. W., "The Effect of Sulfur on the Anodic H.sub.2 (Ni) Electrode in Fused Li.sub.2 CO.sub.3 -K.sub.2 CO.sub.3 at 650.degree. C.", J. Electrochem. Soc. 129 (7) 1441-45 (1982); Tang, T. E., Claar, T. D., and Marianowski, L. G., "Effects of Sulfur-Containing Gases on the Performance of Molten Carbonate Fuel Cells", Interim Report EM-1699 prepared for Electric Power Research Institute by Institute of Gas Technology, February 1981; Sammels, A. F., Nicholson, S. B., and Ang. P. G. P., "Development of Sulfur-Tolerant Components for the Molten Carbonate Fuel Cells", J. Electrochem. Soc. 127, 350 (1980); and Claar, T. D., Marianowski, L. G., and Sammells, A. F., "Development of Sulfur-Tolerant Components for Second-Generation Molten Carbonate Fuel Cells", Interim Report EM-1114, prepared for the Electric Power Research Institute by Institute of Gas Technology, July 1969. A review of the effect of sulfur containing compounds on molten carbonate fuel cells is given in Marianowski, L. G., "An Update of the Sulfur Tolerance of Molten Carbonate Fuel Cells", paper presented at Third Annual Contaminant Control in Hot Coal Derived Gas Streams, Washington, Pa. May, 1983. The expense of using pure hydrogen gas fuels is high as is the expense of sufficient removal of sulfur containing contaminants to a level which maintains satisfactory fuel cell operation with many conventional anode materials.
U.S. Pat. No. 3,431,146 teaches a molten carbonate fuel cell having a fuel electrode of nickel, cobalt, or iron, exhibits at least a temporary increase in power output by addition of hydrogen sulfide to a hydrogen fuel gas. In specific examples, this patent teaches 2.0 volume percent hydrogen sulfide added to hydrogen fuel gas increased power output by 50 percent during a one minute flow of the added hydrogen sulfide.
U.S. Pat. No. 4,247,604 teaches molten carbonate fuel cell porous anodes including a chromium, zirconium, or aluminum surface area stabilizing agent to improve surface area stability of the porous anode during fuel cell operating conditions.